In recent years, remarkable progress has been attained in displays. Various displays such as liquid crystal displays and EL displays have been actively incorporated in an OA apparatus such as a PC and a word processor. Each of these displays has a sandwich structure in which a display element is disposed between transparent conductive films.
Currently, a silicon-based semiconductor film has been used mainly as a switching device of a thin film transistor (TFT) or the like which is used to drive the above-mentioned display. The reason therefor is that, in addition to improved stability and processibility of a silicon-based thin film, a thin film transistor using a silicon-based thin film has advantages such as a high switching speed. Generally, this silicon-based thin film is fabricated by the chemical vapor deposition (CVD) method.
However, in the case of an amorphous silicon-based thin film, there are disadvantages that the switching speed is relatively low and images cannot be displayed when a high-speed animation or the like are displayed. Further, in the case of a crystalline silicon-based thin film, although the switching speed is relatively high, heating at a high temperature of 800° C. or higher, heating by means of a laser or the like is required, and hence, a large amount of energy and a large number of steps are required in production. Although a silicon-based thin film exhibits superior performance as a voltage element, it encounters a problem that its properties change with the passage of time when current is flown.
An oxide semiconductor has attracted attention as a material or the like which is used to obtain a transparent semiconductor film which is superior to a silicon-based thin film in stability and has light transmittance equivalent to that of an ITO film.
However, in a film containing crystals of indium oxide, in particular a polycrystalline film, oxygen deficiency tends to occur easily. It is believed that it is impossible to allow the carrier density to be 2×10+17 cm−3 or less even if the oxygen partial pressure during film formation is increased, an oxidization treatment or the like is conducted. Therefore, no attempt has been made to use this film as a semiconductor film or as a TFT.
Under such circumstances, Patent Document 1 discloses a thin film transistor having a semiconductor layer comprising indium oxide. Specifically, this document discloses a method in which a thin film transistor is obtained by subjecting an indium oxide film to a heat treatment under an oxidizing atmosphere. However, in the case of a thin film formed of indium oxide, performance of the resulting thin film transistor varies depending on heat treatment conditions, oxidizing atmosphere conditions, in particular, on humidity conditions when a heat treatment is conducted in the air, resulting in unstable transistor performance.
On the other hand, Patent Documents 2 and 3 disclose that an amorphous oxide semiconductor can be stably obtained due to the presence of a hydrogen element or a deuterium element in an amorphous oxide semiconductor film. However, since an amorphous oxide semiconductor film has problems that, due to its amorphous nature, a hydrogen element or a deuterium element within the film may be diffused in the air, or water molecules may enter the film from the air, allowing the amount of a hydrogen element in the film to be excessive and hence, making the resulting device to be unstable.
If crystalline indium oxide is used in a semiconductor film, since the resulting semiconductor film is not dissolved in oxalic acid, PAN or the like, which means that the film has etching resistance, there are advantages that a channel-etch TFT can be produced easily. However, it is significantly difficult to allow a crystalline indium oxide film alone to be semiconductive by sufficiently lowering the carrier density thereof. That is, if an indium oxide film is merely crystallized, carriers are generated due to oxygen deficiency or the presence of a positive tetravalent metal oxide which is a coexistent impurity, whereby the oxide indium film may become a conductor. Therefore, a TFT which uses crystalline indium oxide in a semiconductor film has not been fabricated so far.